Macular pigment is a yellow pigment situated in the central portion of the human retina. The absorption spectrum for the pigment has a peak for light of a wavelength of 460 nm and zero for light for a wavelength of 540 nm, so that the pigment absorbs significant amounts of the shorter wavelength light, whilst having little or no effect on light of the longer wavelength.
The highest concentrations of macular pigments are to be found in the region of the retina which has a very high number density of cone receptors, and is coupled with a disproportionately large area of the visual cortex, giving that region a high degree of visual acuity.
It has been proposed that the macular pigment protects the retina against harmful effects of short wavelength radiation, and accordingly much work has been devoted to measuring the optical density, and spatial distribution, of macular pigment in various subjects in order to determine whether there is any correlation between irregularities in the amount of macular pigment present and certain defects.
A flicker photometer is an instrument that enables a subjective measurement of macular pigment density to be made. The flicker photometer projects green and blue light in an alternating sequence into a subject's eye, and the subject is able to vary the relative intensity of light of one of those colours until a minimum or no flickering is perceived.
Photographic methods have also been used to obtain an objective indication of the macular pigment density/spatial distribution, but in order to be effective, have involved dilating the subjects pupil, bleaching photo pigments to minimise their contributions and then photographing the fundus twice, once in blue light and once in green light. Those images are then digitised (if not already captured by a CCD camera), combined in registration with each other, logarithmically transformed and then subtracted.
However, ensuring that the images are precisely registered, is a time consuming step which places high demands on image processing software and hardware.